Since the original works of Zeleny (Zeleny, J., Phys. Rev., 1914, 3, 69-91; Zeleny, J., Phys. Rev., 1917, 10, 1-6) and Taylor (Taylor, G., Pro. R. Soc. A, 1964, A280, 383-397), it has been known that the application of a high electric field to a liquid will cause the liquid to become unstable and to break up into many smaller daughter droplets. It is known that if a liquid effluent is pumped though a capillary nozzle, and the exit of the nozzle is placed in a high electric field relative to the surroundings, the liquid exiting the nozzle will break-up into a continuous stream of charged droplets, as shown in FIG. 1. This process of electrohydrodynamic atomization is commonly referred to as electrospray (Cloupeau, M. and Prunet-Foch, B., J. Aerosol Sci., 1994, 25, 1021-1036).
Electrospray has many practical applications. It has been utilized in the application of thin film coatings, thick film coatings such as electrostatic painting, and powder deposition. Importantly, it is also a practical source of ionization, in which ions present in the liquid are transformed to gas phase ions, through the process of atmospheric pressure ionization. In this configuration, electrospray is often used in combination with the analytical technique of mass spectrometry. Electrospray ionization-mass spectrometry is a method of nearly universal application for chemical analysis, finding wide use in chemical manufacturing, analytical chemistry, environmental chemistry, and perhaps most importantly in the life sciences. Electrospray is currently the method of choice to interface high performance liquid chromatographic (HPLC) separations to mass spectrometry, referred to here, as LC-MS. HPLC is a key tool in separation science, whereby a mixture of components in a liquid phase are seperated, with the mass spectrometry providing high specificity chemical identification. LC-MS plays a central role in pharmaceutical drug discovery and development. Thus practical improvements to the stability, and/or sensitivity, of the electrospray method are of considerable importance.
It is known to those skilled in the art that the stability of an electrospray process is a function of several interdependent parameters, such as:                (1). Nozzle (tip) geometry,        (2) Electric field strength, which is in turn a function of:                    (A) Applied voltage and            (B) Distance to Counter electrode,                        (3) Mobile phase flow rate,        (4) Mobile phase chemical composition.        
Because of the interdependency of these variables, a certain amount of empirical work is required to tune each particular electrospray system for optimal results in each particular application. In most systems, one or more of the foregoing parameters are either fixed or difficult to adjust. In most systems, therefore, the tuning that is required to obtain electrospray stability is generally accomplished by varying and adjusting the electric field strength at the nozzle. This, in turn, requires adjusting either the applied voltage or the distance between the nozzle and counter electrode or mass spectrometer inlet system.
Electrospray systems are generally tuned by one of two different methods. In the first method, the electrospray nozzle is visualized through, for example, a microscope, video camera, etc. and then an operator manually adjusts experimental parameters, such as voltage, distance or both, until a satisfactory spray pattern is achieved. In a second method, the ion current generated by the electrospray process is monitored while the voltage, distance (between the nozzle and counter electrode or mass spectrometer inlet) or both are adjusted. The parameters are adjusted until an ion current of satisfactory magnitude or stability is obtained. Adjustments may be carried out under manual control by an operator, or under electronic (i.e., computer) control for an automatic tuning process. The ion current tuning method is most often employed when an electrospray system is being used as an ionization source in communication with a mass spectrometer.
Both of the foregoing methods have serious limitations. The manual method using visualization of the electrospray nozzle requires constant operator attention and adjustment, and does not respond to varying conditions unless the operator observes and reacts to such changing conditions. Ion current, as used in the second method, on the other hand, is not a completely satisfactory choice upon which to base control, because it is dependent on the chemical nature of the liquid exiting the electrospray nozzle. A change in the chemical composition will change the ion current. This results in a system that must be re-tuned when the chemical composition of the liquid changes.
It has been well established (Cloupeau, M. and Prunet-Foch, B., J. Aerosol Sci., 1994, 25, 1021-1036; Jaworek, A. and Krupa, A., J. Aerosol Sci., 1999, 30, 873-893) that the liquid effluent (the mobile phase) and subsequent spray exiting the nozzle may take on a wide variety of physical forms, or spray modes. Jaworek and Krupa (Jaworek, A. and Krupa, A., J. Aerosol Sci., 1999, 30, 873-893) identified ten distinct spray modes, each with definable time-dependant morphological characteristics. The specific spray mode obtained depends strongly upon the geometry of the nozzle, the strength and shape of the electric field, and the mobile phase chemical composition. The spray modes are particularly sensitive to the mobile phase surface tension, viscosity, and electrical conductivity (Grace, J. M. and Marijnissen, J. C. M., J. Aerosol Sci., 1994, 25, 1005-1019). FIG. 2 shows the basic relationship of the electrical potential and flow rate for the most common electrospray modes for an aqueous based mobile phase. The most commonly encountered modes are shown in FIGS. 3 through 8 and are referred to as: dripping mode, spindle mode, pulsed cone-jet mode, cone-jet mode, and multi-jet mode. Each mode will generate a given distribution of droplet sizes, with each droplet carrying a distribution of electrical charge. The dripping mode typically generates the largest observable droplets, producing drops that can be millimeters in diameter. These droplets can be larger in diameter than the nozzle itself. The cone-jet and multi-jet modes produce the smallest droplets having the highest charge-to-mass ratio. The cone-jet and multi-jet modes are capable of producing nearly monodisperse droplets, having a narrow distribution in both diameter and charge state. Droplet diameters for these modes can be sub-micrometer, much smaller than the diameter of the nozzle itself. Some modes, such as the spindle mode and pulsed cone-jet mode, generate droplets of a large distribution in size and charge, which is not desirable for many applications. These modes also exhibit a pulsing or oscillatory behavior, which can range in frequencies from tens of Hertz to hundreds of Kilohertz. The combination of a wide size distribution along with pulsing behavior is undesirable for many applications. In mass spectrometry, for example, spray pulsing can create poor reproducibility in signal measurement and waste sample, since ion current is not being generated 100% of the time. Large droplets are also known to contribute a significantly to the total ion current yielding a high degree of non-specific “chemical noise” to the mass spectrum.
Of the possible spray modes, the most desirable for many practical applications, including mass spectrometry, is the cone jet mode, as shown in FIG. 7. The cone-jet mode generates a fine aerosol of small, nearly mono-disperse droplets, 100% of the time. Furthermore such droplets are also known to have the highest possible charge-to-mass ratio. Such small, highly charged droplets are known to yield optimal sensitivity for analysis by mass spectrometry. Considerable interest in the prior art has been spent on the characterization of the individual modes and the droplet size distributions and ion signal intensities that result from such modes, with particular attention being paid to the cone-jet mode. A number of diagnostic techniques are available for such characterization.
The simplest method for determination of the spray mode is to utilize continuous illumination from a strong light source and observe the shape of the spray with an optical microscope using either transmitted light or scattered light illumination, as shown in FIG. 8. This method has been incorporated into a wide variety of experimental apparatus and is available commercially from a number of vendors (Product Literature, New Objective, Inc. 2002). For example Juraschek et al. (Juraschek, R., Schmidt, A. et al., Adv. Mass Spectrom., 1998, 14, 1-15) used this method to observe the spray mode in relation to the ion current as monitored by mass spectrometry. A relationship between ion intensity and the spray mode was established, with the axial cone-jet mode showing optimal results. Zhou et al. (Zhou, S., Edwards, A. G. et al., Anal. Chem., 1999, 71, 769-776) utilized laser illumination and fluorescence imaging detection to probe the fluorescence characteristics present in the spray. They were able to measure the pH of the plume for the cone-jet mode in a sheath gas assisted spray.
Another common method for characterization is imaging based on (nanosecond pulse) flash illumination, replacing the continuous light source. Zeleny (Zeleny, J., Phys. Rev., 1917, 10, 1-6) used a flash photographic system, that became the basis for much subsequent work, although the details of the flash electronics and imaging have since been vastly improved and modernized. Cloupeau and Prunet-Foch (Cloupeau, M. and Prunet-Foch, B., J. Aerosol Sci., 1994, 25, 1021-1036) utilized flash strobe imaging with an illumination time on the order of 20 nanoseconds. In addition, a focused laser beam intersected the droplet meniscus and a photo-detector was used to determine the timing of the electronic flash. The output of the photo-detector also yielded frequency information for the study of pulsating modes. Tang and Gomez (Tang, K. and Gomez, A., Phys. Fluids, 1994, 6, 2317-2332; Tang, K. and Gomez, A., J. Colloid and Interface Sci., 1995, 175, 326-332) utilized a Xenon nanosecond flash lamp to illuminate the cone-jet region in a CCD Camera based “shadowgraph” imaging system that was used to obtain digital images suitable for computer acquisition. This system was utilized to ensure that the spray was operating in a stable cone-jet mode for subsequent measurements. Strobed imaging systems such as these can determine the nature and stability of the cone-jet, and give direct size measurements of droplets typically larger than approximately 5 to 10 μm.
A common non-imaging means for spray characterization is the use of phase Doppler anemometry (PDA) (Naqwi, A., J. Aerosol Sci., 1994, 25, 1201-1211). PDA can determine both the velocity and size of a droplet as it passes though a detection zone. The measurement is made from detection of the light scattered by the droplet as it crosses interference fringes, which define the detection zone, created by the intersection of two focused laser beams. Three photodetectors detect the intensity and phase of the scattered light, and through a differential calculation, the size of the droplet is determined. Gomez and Tang used PDA to determine the fission characteristics of droplets produced by electrospray for heptane (Gomez, A. and Tang, K., Phys. Fluids, 1994, 6, 404-414; Tang, K. and Gomez, A., Phys. Fluids, 1994, 6, 2317-2332) and water (Tang, K. and Gomez, A., J. Colloid and Interface Sci., 1995, 175, 326-332) for the cone-jet mode. Olumee et al. (Olumee, Z., Callahan, J. H. et al., J. Phys. Chem., 1998, 102, 9154-9160) used PDA to determine droplet dynamics for methanol-water mixtures. The use of PDA alone is unable to distinguish a particular spray mode since it only samples a small percentage of the total droplets generated by the spray at one particular volume in space. For example, if the PDA detection zone is positioned off-axis to the nozzle, it will only detect the smaller droplets, and miss the larger droplets of the spindle and pulsed cone-jet modes.
Other methods have been used to either measure droplet size or to determine other spray characteristics using non-optical methods based on mobility. De Juan and Fernandez De La Mora (De Juan, L. and Fernandez De La Mora, J., J. Colloid and Interface Sci., 1997, 186, 280-293) utilized a differential mobility analyzer in conjunction with an aerodynamic size spectrometer to measure the charge and size distributions for electrospray drops for a number of organic solutions based on benzyl alcohol and dibutyl sebacate. The differential mobility analyzer was used to determine the charge on the droplet in conjunction with a microscope imaging system to monitor the spray mode exiting the capillary nozzle. Droplets passing though the mobility analyzer entered the aerodynamic spectrometer for size analysis. The aerodynamic spectrometer determines the diameter of a droplet from measuring the velocity of the droplet as it enters a supersonic jet. This method is of limited application to mass spectrometry since the measurement is a destructive technique and is limited to mobile phases of limited volatility. As with PDA, these non-optical methods are not directly capable of determining the particular spray mode.
Oscillations and pulsation in various spray modes have been detected by directly monitoring the spray current by a number of research groups including Juraschek and Rollgen (Juraschek, R. and Rollgen, F. W., Int. J. Mass Spectrom., 1998, 177, 1-15) and Vertes et al. (Carney, L., Nguyen, A. et al., Proceedings of the 49th Annual Conference on Mass Spectrometry and Allied Topics, 2001). In this configuration, as shown in FIG. 9A and FIG. 9B, the spray current supplied to the nozzle (FIG. 9A) or that detected on the counter electrode (FIG. 9B) is sent to an oscilloscope for frequency analysis. Juraschek and Rollgen (Juraschek, R. and Rollgen, F. W., Int. J. Mass Spectrom., 1998, 177, 1-15) observed low (10-50 Hz) and “high” frequency (1.5 to 2.5 kHz) pulsation and determined the dependence of the frequency on flow rate and mobile phase composition. Ion signal intensities were monitored simultaneously by mass spectrometry. The highest signal intensities were observed for the cone-jet mode. Even though the authors went to extensive efforts to maintain a high bandwidth detection system, this method is limited to the observation of only relatively low frequency oscillations of the larger droplets produced by the spindle and pulsed cone-jet modes. The current measurement technique is unfortunately inherently limited in bandwidth, and is apparently unable to distinguish the high frequency (>50-100 kHz) events. The reason is that the higher frequency events carry less current, typically in the picoamp range, and therefore require greater gain in the detection electronics. The greater gain requirements of the current amplifier serve to limit the bandwidth. System bandwidth is further limited by the presence of stray capacitance within the capillary nozzle, and between the capillary nozzle and counter-electrode. Although the authors suggest that this method obviates the need to determine the spray mode with an optical microscope, the highest oscillation frequencies observed by this technique were well below 5 kHz. It is known that higher pulsing frequencies are both possible and very likely to occur. This method leaves the spray insufficiently characterized.
For a given mobile phase composition, optimizing the spray is usually a matter of adjusting the flow rate and electric field potential (voltage) to generate and maintain the desired spray mode, which is often the cone-jet mode. Mobile phase composition is typically not a freely adjustable parameter, since the intended application usually dictates a specific range of chemical composition. In LC-MS, for example, the mobile phase typically consists of a mixture of acetonitrile and water, with a trace quantity (0.001 to 1%) of acid such as formic, acetic, or trifluoroacetic acid. When using electrospray for thin-film deposition the chemical composition of the mobile phase is similarly fixed. Such fixed chemical composition will only yield a cone-jet mode for a specific nozzle diameter, over a limited range in applied voltage and flow rate. In mass spectrometry, a well-established method for voltage optimization, presumably to the cone-jet or similar mode, is to observe the strength of the ion signal detected by the mass spectrometer while adjusting the voltage. A number of commercial instruments are capable of automatically tuning the spray voltage based on the highest ion intensity as observed by mass spectrometry.
Optimization methods based on either total spray current or specific ion current, such as that provided by mass spectrometry, yield a signal which is highly dependant on the chemical composition of the mobile phase. It is desirable to have a tuning method that is completely independent, if not orthogonal to the spray or ion currents generated by the spray.
Ion or spray current optimization methods fall short in many circumstances. Often, especially when operated with sample delivery by liquid chromatography, there is insufficient ion intensity to make a meaningful adjustment. Or one incorrectly chooses and maximizes an ion signal that relates to a noise peak, thus actually decreasing the amount of observable analyte ion signal by maximizing background noise. The situation in LC-MS is further complicated by the fact that the chemical composition of the mobile phase changes significantly when operated under conditions of gradient elution. In gradient elution chromatography, the mobile phase composition is typically ramped from one mobile phase composition to another. For example, at the start of an analytical run, the mobile phase may start with a composition of 5% Acetonitrile, 95% water and be reversed to 95% acetonitrile, 5% water at the end. If the spray voltage were adjusted to generate a cone-jet mode at the start of this run, then by the time the run is finished the mode is most likely to be in the unstable multi-jet mode due to the much lower surface tension of the 95% mixture of acetonitrile. Likewise if the voltage were adjusted for the cone-jet mode at the end of the run, the mode at the start would be the dripping or spindle mode. In practice one often makes a compromise where the cone-jet mode is maintained at the middle of run, sacrificing performance at the start and the end. Thus not only are the conditions for the cone-jet mode different at the ends of the run, they are continuously changing during the run itself. Thus during the run, the applied spray voltage must also change if the cone-jet mode is to be maintained during the gradient.
Flow rate is another parameter that is often not readily adjustable. In LC-MS for example, the mobile phase flow rate for a given experiment is often fixed within a specific range and is determined by the type of chromatography being preformed. It is also common that in combination with gradient chromatography that the flow rate of the mobile phase can change, resulting again in a need to adjust the spray voltage to maintain the cone-jet mode.
Prior attempts to deal with this unfavorable situation have been primarily concerned with the electrospray nozzle geometry. Most prior art focus on the use of sheath gases or liquids, the size and sharpness of the capillary spray nozzle, or using a combination of both. Rather than attempting to determine and control the specific spray mode, most of the methods attempt to eliminate the undesirable aspects of the large droplets generated by certain spray modes.
U.S. Pat. No. 4,935,624, teaches that the application of a heated sheath gas surrounding the capillary nozzle can be beneficial for sensitivity. U.S. Pat. No. 5,349,186 teaches that specific heating of the sheath gas can be beneficial, especially when spraying liquids composed primarily of water. These patents relate their increase in performance due to a decrease in droplet size when the sheath gas is present. U.S. Pat. Nos. 5,306,412 and 5,393,975 both teach the use of a triple layer nozzle, in which both liquid and/or gas can be co-axially applied to the capillary nozzle. Again, through the addition of sheath gas, the effects of modes that create larger sized droplets can be reduced. In addition, a sheath liquid can be used to aid in control of the mobile phase surface tension. Thus by adding a chemical modifier to the mobile phase to reduce surface tension, the droplet size is reduced, and sensitivity improves. A similar method is disclosed by Smith et al. (U.S. Pat. No. 5,423,964) to help deal with the uncertain chemistry when using electrospray to couple capillary electrophoresis with mass spectrometry.
U.S. Pat. Nos. 5,115,131; 5,504,329; and 5,572,023 show that spray performance, and hence sensitivity, can be improved if the size of the capillary nozzle is reduced. U.S. Pat. No. 5,504,329 shows that a wide range of chemical compositions may be suitably sprayed if the size of the nozzle is reduced to micrometer dimensions. The inventors relate the improvement in sensitivity to a reduction in droplet size caused by reductions in both flow rate and the diameter of the nozzle.
Moon et al. (U.S. Pat. No. 6,245,227 B1 and U.S. patent application Ser. No. 2001/0001474 A1) shows the use of lithographic fabrication techniques on planar substrates to fabricate a controlled nozzle geometry can be beneficial for low flow rate electrospray operation. The method of Moon et al. introduces the use of a secondary substrate voltage or voltages to control and enhance the strength of the electric field at the exit of the nozzle. In their configuration, the voltage applied to the nozzle is different from that applied to the mobile phase. The increase in field strength presumably generates smaller droplets for enhanced sensitivity. The inventors describe a system in which a spray attribute sensor or sensors integral to the nozzle substrate, would be used to control the nozzle voltage. Moon, et al. do not disclose how such a system might be implemented, constructed, or used for the determination and control of spray modes.
In US patent application US 2002/0000517 A1, Corso et al. disclose the fabrication and use of similar nozzles for improved sensitivity. Corso et al. also describe an increase in electrospray signal relating to the number of spray jets emanating from a single nozzle while in the multi-jet mode. While the inventors observe an increase in signal for each jet formed on the surface of the nozzle for a fixed mobile phase chemistry, they do not teach how such multiple jets may be actively controlled on a single nozzle. To overcome this limitation, the inventors resort to the fabrication and use of multiple nozzles, each supporting a single cone-jet mode.
For applications where the chemical composition of the mobile phase composition or flow rate can change, there is a need to have an electrospray based source that is capable of performing well under varying experimental conditions. Ideally this would be a system by which a particular spray mode can be established and maintained, regardless of the chemical composition or flow rate of the mobile phase. Furthermore it is desirable to have a system that can self optimize and self-correct in a manner which is completely independent of the ion current generated by the spray. None of the prior art provides a system that is self-tuning and capable of establishing and maintaining a given spray mode for varying mobile phase composition or flow rate.